AVMutableVideoComposition和AVVideoCompositionCoreAnimationTool颜色故障

Question

我正在尝试使用以下代码将图像添加到视频中：

    AVURLAsset* videoAsset = [[AVURLAsset alloc]initWithURL:[self assetURL] options:nil];
AVMutableComposition *mixComposition = [[AVMutableComposition alloc] init];
AVMutableCompositionTrack *videoTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
[videoTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, videoAsset.duration)
                    ofTrack:[[videoAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0]
                     atTime:kCMTimeZero error:nil];

AVMutableCompositionTrack *audioTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];
[audioTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, videoAsset.duration)
                    ofTrack:[[videoAsset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0]
                     atTime:kCMTimeZero error:nil];

AVMutableVideoCompositionInstruction *mainInstruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];
mainInstruction.timeRange = CMTimeRangeMake(kCMTimeZero, videoAsset.duration);
AVMutableVideoCompositionLayerInstruction *videolayerInstruction = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:videoTrack];

AVAssetTrack *videoAssetTrack = [[videoAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0];
[videolayerInstruction setTransform:videoAssetTrack.preferredTransform atTime:kCMTimeZero];
mainInstruction.layerInstructions = [NSArray arrayWithObjects:videolayerInstruction,nil];

AVMutableVideoComposition *mainCompositionInst = [AVMutableVideoComposition videoComposition];
CGSize naturalSize = CGSizeApplyAffineTransform(videoTrack.naturalSize, videoTrack.preferredTransform);
mainCompositionInst.renderSize = naturalSize;
mainCompositionInst.instructions = [NSArray arrayWithObject:mainInstruction];
mainCompositionInst.frameDuration = CMTimeMake(1, 30);

CGFloat ratio = naturalSize.width / rectVideo.size.width;
CALayer *parentLayer = [CALayer layer];
CALayer *videoLayer = [CALayer layer];
parentLayer.frame = CGRectMake(0, 0, naturalSize.width, naturalSize.height);
videoLayer.frame = CGRectMake(0, 0, naturalSize.width, naturalSize.height);
[parentLayer addSublayer:videoLayer];


for(DraggableImageView *iv in imageViews){
    CALayer *overlayLayer = [CALayer layer];
    [overlayLayer setContents:(id)[[self normalizedImage:iv.image] CGImage]];
    overlayLayer.frame = CGRectMake(iv.frame.origin.x * ratio, (rectVideo.size.height - iv.frame.origin.y - iv.frame.size.height) * ratio, iv.frame.size.width * ratio, iv.frame.size.height * ratio);
    [overlayLayer setMasksToBounds:YES];
    overlayLayer.opacity = 0;
    overlayLayer.masksToBounds = YES;

    CGFloat duration = CMTimeGetSeconds(iv.endTime) - CMTimeGetSeconds(iv.startTime);
    CGFloat start = CMTimeGetSeconds(iv.startTime);
    if(duration > CMTimeGetSeconds(videoAsset.duration)){
        duration = CMTimeGetSeconds(videoAsset.duration);
        start = 0;
    }
    CGFloat fadeInSeconds = iv.fadeInSeconds;
    CGFloat fade = fadeInSeconds / duration;
    CAKeyframeAnimation *anim = [[CAKeyframeAnimation alloc] init];
    anim.keyPath = @"opacity";
    anim.values = @[@0, @1, @1, @0];
    NSMutableArray *keyTimes = [NSMutableArray new];
    [keyTimes addObject:@0];
    [keyTimes addObject:@(fade)];
    [keyTimes addObject:@(1.0 - fade)];
    [keyTimes addObject:@1];
    anim.keyTimes = keyTimes;
    anim.duration = duration;
    anim.beginTime = AVCoreAnimationBeginTimeAtZero + start;
    [overlayLayer addAnimation:anim forKey:@"anim"];

    [parentLayer addSublayer:overlayLayer];
}


mainCompositionInst.animationTool = [AVVideoCompositionCoreAnimationTool
                             videoCompositionCoreAnimationToolWithPostProcessingAsVideoLayer:videoLayer inLayer:parentLayer];

NSURL *exportURL = [self exportUrl];

exportSession = [[AVAssetExportSession alloc] initWithAsset:mixComposition presetName:AVAssetExportPresetHighestQuality];
exportSession.outputURL = exportURL;
exportSession.videoComposition = mainCompositionInst;
exportSession.outputFileType = _recordSession.fileType;
exportSession.timeRange = CMTimeRangeMake(kCMTimeZero, videoAsset.duration);
exportProgressTimer = [NSTimer scheduledTimerWithTimeInterval:.1 target:self selector:@selector(exportDidProgress:) userInfo:nil repeats:YES];
[exportSession exportAsynchronouslyWithCompletionHandler:^{
    switch (exportSession.status) {
            case AVAssetExportSessionStatusCompleted:
                [self savingComplete:exportSession.error];
                break;
            case AVAssetExportSessionStatusFailed:
                [self savingComplete:exportSession.error];
                break;
            case AVAssetExportSessionStatusCancelled:
                break;
            default:
                break;
        }
}];

此代码在iPhone SE上效果很好，但在iPad Air 2上，在视频的右侧显示了该色差（放大的屏幕截图）：

此故障不是一成不变的，它会垂直变化（大小和y位置）。 我认为这不是视频大小的问题（众所周知，视频大小不能被4或16分割时，边缘会出现绿线）。该视频的大小为1080x1920。

有什么想法吗？